Notes on Cellular Processes

The Cell Membrane

Passive processes: do not require energy on the part of the organism.
Difussion: movement from an area of higher concentration to an area of lower concentration.

Osmosis: a special type of difussion through a semi-permeable membrane. The most relevant example of the semi-permeable membrane is the plasmalemma. Any time material crosses the cell membrane passively it is by the process of osmosis. Water, as well as solutes, moves across the membrane! When the concentration of solutes inside and outside of the cell is not equal both water and solutes may move across the membrane in order to establish equilibrium. It is not always possible to establish equilibrium for all solutes. Remember: the reason we are organisms is that we organize or concentrate parts of our environment within ourselves. The following terms are frequently used in reference to RBCs when placed in solutions of varying tonicities (solute concentrations):
 

Hypotonic Solution: contains less solute concentration than the cell; water will tend to enter the cell and swell it. This can lead to rupture of the cell which is called hemolysis.

Isotonic Solution: the solute concentration is equal both in and out of the cell. No net movement results.

Hypertonic Solution: contains more solute concentration than the cell; water will tend to leave the cell and the cell will shrink. This is called crenation.
 

Facilitated Difussion: a type of difussion wherein the movement of solutes takes place at rates higher than would be expected. The membrane must be "facilitating" the movement of solutes in or out (speeding up the process)

Dialysis: this passive process is the separation of molecules on the basis of size alone.

Active Processes: movement from an area of lower to higher concentration at the expense of energy by the organism doing the work.
 
 
Active Transport: carrier molecules in the cell membrane bring solutes in against a concentration gradient. This leads to a higher concentrations of certain solutes within the cell. Carriers are all believed to be proteins. Not all solutes have corresponding carrier molecules. Once molecules are within the cell, the same carriers are responsible for keeping them there against the natural tendency of osmosis to return them to the exterior.

Cytosis: the engulfing of food particles or liquids by the cell membrane with the expense of energy. Phagocytosis is the name given to "cell eating" wherein a small food particle or cellular debris is ingested, while pinocytosis is the term given to the uptake of fluid. These processes result in the formation of food vacuoles and pinocytic vesicles, respectively.

Cell Division

Cell division can be divided into Mitosis and Meiosis. The former is occasionally called somatic cell division while the latter is referred to as reduction cell division.

The consequences of mitosis are that one diploid (2n) cell divides to form two diploid cells. There is no genetic recomination (although mutation may occur). The duplication of genetic material needed to accomplish this occurs between divisions in interphase. Because of this and the fact that most cellular activities take place in the non-dividing cell, interphase should NOT be called the "resting stage". In humans, the nucleus of normal, somatic cells contains 23 pairs of chromosomes. The total number of chromosomes is 46 and the ploidy number (n) is 23.
 

  • Stages of Mitosis
  • Statges of Meiosis